This invention relates to electrical machines using permanent magnets and in particular to axial field motor/generators that are robust and include design features for high efficiency in generating energy or developing torque. The optimized modular design allows the electrical machine to be versatile and incorporated in a variety of applications from transportation and power generation to machinery and robotics.
Axial field electrical machines differ from drum type machines by the ability to stack a series of similar modules along an axis to achieve multiple levels of power generation or torque development. In a typical module of an axial field electrical machine, a magnet assembly has two sub-assemblies that are spaced apart to create an axial flux gap in which a disk-like conductor assembly is centrally positioned. Movement of the conductor assembly relative to the magnet assembly induces a current in the conductor or conductors that in part make up the conductor assembly. Conversely, running a current through the conductors generates an electromagnetic field interacting with the magnetic field of the permanent magnets inducing a torque and a relative motion between the magnet assembly and the conductor assembly.
Therefore, in the description of the preferred embodiment, it is to be understood that the terms rotor or stator apply to the described embodiment and that either the magnet assembly or the conductor assembly may rotate. In certain applications, for example, in a wind turbine with counter rotating blades, both the magnet assembly and the conductor assembly may counter-rotate.
In optimizing the electrical machine of this invention, a novel counter-intuitive design and construction has been devised for the conductor assembly. Typically, the conductor assembly is constructed as a disk with radial conductors arranged to pass between a pair of multiple pole magnet subassemblies. In this configuration, the segments of the conductors are normal to the direction of motion of magnetic fields for maximized effect in instantaneous voltage or torque generation.
However, the traditional designs do not optimize other criteria that contribute to the overall efficiency and performance of an electrical machine having a high power-to-weight ratio. The flux or field strength of the permanent magnets is not only dependent on the strength of the magnets, but the width of the air gap between displaced poles. In general, the narrower the air gap, the greater the field strength for given magnets. In addition to providing clearance large enough for reliable mechanical movement of the magnet assembly relative to the conductor assembly, the composite circular or annular disk forming the conductor assembly must be sufficiently robust to absorb the torque whether the mode of the electrical machine is operationally a motor or a generator. In addition to the structural integrity, it is desirable to maximize the mass of the conductors exposed to the field between the magnet poles. In addition to maximizing the power or torque, the resistance is minimized, thereby reducing the generated thermal energy during operation.
In high power, high torque electrical machines of the type described, high energy permanent magnets are preferred. Low-cost, standard NdFeB “rare earth” magnets are temperature sensitive and are subject to degradation even at the moderate temperatures generated by the conductor windings. Design strategies that increase the conductor mass to reduce the resistance and enlarge the heat sink effect of conductor circuits can be coupled with magnet cooling strategies for greatly improved performance. By confining the conductors of the conductor assembly to a flat annular ring in the path of the field, the effective length of the conductors for power or torque generation is maximized while the ineffective segments extraneous to the field that nevertheless generate heat are minimized.
The conductor assembly of the invented electrical machine is optimized by a planar, double-helix annular conductor path with conductor segments that are involute or arc shaped. Optimally, the midsection of each involute or arcuate conductor segment is angled approximately 45° from the radial. The involute shaped conductor segments allow adjacent conductor segments to be more densely packed with uniform minimal space between adjacent segments. The involute configuration allows for equidistant spaced curved lines on a ring defined by concentric diameters and a base pitch circle. The arc shaped segments closely approximate the involute conductor configuration and may have fabrication advantages with only minor effect on density and the uniformity of the insulating space between segments. The arc shaped segments are preferred when the number of magnets in a circular series exceeds ten. Additionally, with the slight crescent shape of the insulator filled space between conductor segments, greater strength at the peak stresspoint may be obtained. Alternately, small cooling channels may be provided between the conductor segments.
Although the novel conductor assembly can be utilized with a rotary field assembly that includes a stationary field winding to generate the electromagnetic fields in a stator for operation as a variable speed motor or variable voltage generator, it is to be understood that the preferred field assembly is a magnet assembly that includes permanent magnets as the sole or primary means of generating the magnet flux that co-acts with the conductor assembly.
The magnet assembly preferably includes at least two sub-assemblies, each having a retainer structure for holding a series of permanent magnets around a circular locus with a common axis to the discoidal conductor assembly. The series of permanent magnets in each sub-assembly radially align and the two sets of opposed magnets align with the flat, double helix flat ring of the conductor or conductors when the conductor assembly is positioned between the two sub-assemblies of the magnet assembly.
The permanent magnets in the magnet set may be square, circular or other shape that is preferably off-the-shelf to minimize material costs. The arrangement of square magnets in the retainer structure may be corner to corner in a diamond pattern or side by side in a band. The retainer structure is fabricated of a magnetically inert substance such as aluminum and includes a high strength perimeter band when the magnet assembly is configured as a rotor.
When the magnet assembly is configured as a rotor, the electrical machine is advantageously constructed as a versatile brushless motor-generator. The modules may be combined on a common axis to compound the generated power or torque. These and other features are described in greater detail in the sections that follow.